It is known to limit the rate at which successive attempts may be made to complete all or a part of a call to a particular called customer, and a method is disclosed in U.S. Pat. No. 4,224,479 (Kenneth E. Crawford) in which a subsequent attempt is prohibited within a predetermined time interval (gapping interval) from the last earlier allowed attempt.
Crawford's method results in a carried rate approaching one call per gapping interval only for very high offered calling rate of the order of 20 calls per gapping interval. Moreover, at offered calling rates of the order of one call per gapping interval, Crawford's method results in a significantly reduced carried rate even though the switching system can handle calls at such offered rates. Consequently, to avoid significant loss of revenue it is necessary to remove the call control or change the gapping interval when the offered calling rate drops, and such a decision is usually taken by a traffic control manager.